Here is a brief description of what has changed and how it affects some devices.
Is it possible to merge/install split APK files (AKA "app bundle"

, on Android device itself, without root?
...The questions
What is the way to merge those all into one APK file?
Is it possible to install split APK files without root and without PC ? This was possible in the past on backup apps such as Titanium, but only on normal APK files, and not app bundle (split apk).
EDIT: I've set a bounty. Please, if you know of a solution, show it. Show something that you've tested to work. Either of merging split APK files, or installing them , all without root and right on the device.
EDIT: Sadly all solutions here didn't work, with or without root, and that's even though I've found an app that succeeded doing it (with and without root), called "SAI (Split APKs Installer)" ...
https://stackoverflow.com/questions/55212788/is-it-possible-to-merge-install-split-apk-files-aka-app-bundle-on-android-d
Searching for work-arounds was pretty much a waste of time. It seems that APKs are going the way of P2P, Napster, torrents,,,